18-Annulene chemical shifts

Annulene satisfies the Huckel (4n+2) tt electron rule for aromaticity and many of its proper ties indicate aromaticity (Section 11 20) As shown in Figure 13 10a [18]annulene contains two different kinds of protons 12 he on the ring s periphery ( out side ) and 6 reside near the middle of the molecule ( inside ) The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the NMR spectrum The outside protons have a chemical shift 8 of 9 3 ppm which makes them even less shielded than those of benzene The six inside protons on the... 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

A bispyridine derivative of dibenzo-hexaaza[18]-annulene 236 was prepared by Bell and Guzzo.275 Fur an building blocks were used in the syntheses of the diatropic compounds 237 and 238.276-278 The chemical shifts of the indicated hydrogens show that the diamagnetic ring current decreases in the order 237 > 238 (Scheme 83). 

H-Chemical shifts of cyclododeceno[c]furan (an annulated [12]-annulene) and cyclotetradeceno[c]furan (an annulated [14]annulene) have been calculated. Remarkably stable compounds result when a carbonyl... 

The PMR spectra of substituted derivatives, however, provide additional evidence for the structure of bridged [12]annulenes. A comparison of 92a and 92b reveals a normal control of the cyano group over the chemical shift of the neighboring protons (A = 1.0 ppm), the influence on the remote pyrrolic protons being small (A = 0.11-0.39 ppm). Hence diazapyracylenes should be composed of loosely connected closed-shell moieties (formula 97a) the influence of the substituents on the chemical shift being limited to the substituted part of the molecule.112... 

The diatropism of diazapyraceheptylenes (106), which follows from a comparison of the PMR spectra with that of 105a (Table VI), may be due to the effect of the cyano groups on the electronic configuration of the 7r-system. The chemical shift difference between the inside and outside protons clearly reveals the expected diamagnetic ring current in the annulene derivatives (110). The coupling constants of the seven-membered rings of 110 show that this part of the molecule is planar and delocalized. 

Table 6 H NMR Shifts of Aza[19]annulene (77) and Aza[21]annulene (81) <78AHa23)55) Compound Chemical shifts (p.p.m.)...

Strong evidence in confirmation of the above explanation of the chemical shifts of aromatic hydrogens is provided by a study of the cyclic conjugated polyene [18]annulene, which has hydrogens both inside and outside the ring ... 

We have mentioned already (Section 22-3C) the large differences in nmr chemical shifts between the inside and outside hydrogens of [18]annulene —a substance which with 18 tt electrons should be aromatic by the 4n + 2 rule. These differences are observed only at low temperatures. The proton nmr spectrum of [18]annulene at room temperature is a single resonance, which indicates that the inside (Ha) and outside (Hb) hydrogens are equilibrating rapidly. This can take place only if cis-trans interconversion occurs about the double bonds (marked c and t) ... 

Parenthesized values denote differences in chemical shift between a given annulene (123 or 126) and its acyclic model. c Recorded in THF-d6 for 123a and in CDCla for 123c,eji,g, 126a,c,d,f,g. 

---